The invention relates generally to networking, and more particularly to cell-based transmission scheduling for virtual circuits.
In complex networks, cells are transmitted to physical channels (or “ports”) over a virtual circuit according to traffic parameters of the virtual circuit. One example of such traffic parameters are those specified in the “The ATM Forum Technical Committee Traffic Management Specification Version 4.1”, The ATM Forum, March 1999. An ATM virtual circuit connection may characterize its traffic by using source traffic descriptions, which attempt to capture the cell inter-arrival pattern for resource allocation. Once such traffic descriptor is Peak Cell Rate (PCR), which represents the minimum spacing between cells, and therefore the peak emission rate of the source. The PCR is expressed in cell/seconds.
ATM supports a quality of service required by an application through the selection of an appropriate service category. The services offer different QoS commitments in terms of delay and loss tolerance. The services differ in how the network allocates bandwidth and applies different traffic management functions. The service categories include constant bit rate (CBR), variable bit rate (VBR) and unspecified bit rate (UBR). For CBR and VBR, bandwidth is allocated for the duration of the connection. In contrast, UBR services target for use bandwidth that becomes dynamically available as connections go idle.
The CBR service provides a connection with dedicated bandwidth providing extremely low probability of cell loss, as well as low and predictable delay. The inter-arrival time between two cells is constant and can be characterized as a minimum cell inter-arrival, which corresponds to a known PCR.
The VBR service category is mainly intended for more efficient support of applications that have known or predictable bursty traffic characteristics. The VBR traffic can be characterized by a sustained cell rate (SCR) as well as a PCR. The SCR is measured over a defined period and represents the average transmission rate. The VBR service can be further divided into two subcategories based on delay requirements, the real-time VBR (rt-VBR) and non-real-time VBR (nrt-VBR). The rt-VBR has strict end-to-end delay requirements, whereas the nrt-VBR does not guarantee any delay bounds.
A UBR virtual circuit can have a priority determined by the weight associated with such VC on a given port.
Ports can be characterized in terms of the maximum rates at which they are capable of transmitting.